As electronic apparatuses have been reduced in size and weight, various devices tend to be manufactured in thin-film processes. In this trend, a planar magnetic device (planar inductor) having an external-iron structure has been proposed as magnetic devices, such as inductors (reactors), transformers, and magnetic heads, in addition to the structure in which wire is wound around a known bulk magnetic material. In the external-iron structure, a planar coil having a spiral shape or a zigzag (meander type) pattern is coated with a magnetic material. It is thus being attempted to miniaturize devices (see, for example, Non-Patent Document 1).
On the other hand, a technical demand is being increased that devices are operated at an operation frequency on the order of MHz or more to achieve small lightweight electronic apparatuses, as in, for example, a DC-DC converter for small electronic apparatuses. Such an electronic apparatus includes a high-frequency inductor as a key component. The high-frequency inductor requires:
(1) being thin and small;
(2) having superior frequency characteristics; and
(3) having an appropriate power capacity.
In general, practically used miniature inductor elements include a type in which wire is coiled around bulk ferrite material and a type produced by integrally firing a coating ferrite material and a coating conductive material (see, for example, Patent Document 1). The former increases the proportion of degraded portions at the surface to total volume as the bulk ferrite core is reduced in size and thickness, and accordingly, the characteristics, such as magnetic permeability, are degraded. Consequently, it becomes difficult to achieve a low-loss, high inductance inductor element. The latter is produced by patterning a coli in a spiral or toroidal shape, applying ferrite so that the soft magnetic material can be excited by the coil, and finally firing the coil and the ferrite. For example, a toroidal inductor is manufactured through the step of alternately patterning ferrite and a conductor by coating.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-299120
Non-Patent Document 1: IEEE Trans. Magn. MAG-20, No. 5, pp. 1804-1806.
However, soft magnetic materials used in the known inductors generally have low magnetic permeability, and accordingly the inductors do not easily produce high inductance unfortunately. However, if a large amount of magnetic material is used to compensate the above disadvantage in a magnetic device, such as an inductor, it is limited to reduce the thickness of the magnetic device. This makes it difficult to miniaturize apparatuses by high-densely packaging components.